ClearQuip
ClearQuip · Outdoors & weather

Wind chill calculator and heat index calculator

Feels-like temperature · NWS formulas · °F or °C

Work out the feels-like temperature. Cold air plus wind gives a wind chill, and hot air plus humidity gives a heat index. Both use the same formulas as the US National Weather Service.

Inputs

For wind chill

For heat index

Result

Enter temperature plus wind speed (cold) or humidity (hot).

Disclaimer: An estimate using NWS formulas. Direct sun, activity level, clothing and health all change how weather affects you. Follow official weather warnings.

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How to use the wind chill and heat index calculator

  1. Choose Fahrenheit/mph or Celsius/km/h.
  2. Enter the air temperature from a forecast or thermometer.
  3. In cold weather, enter the sustained wind speed. In hot weather, enter the relative humidity. You can fill in both; the tool uses whichever applies to the temperature.
  4. Press Calculate. The result says whether a wind chill or a heat index was used. In hot weather it also shows the NWS risk level (caution, extreme caution, danger or extreme danger).

How it's calculated

Wind chill (NWS, 2001). Valid at or below 50 °F with wind of at least 3 mph. T is in °F and V is in mph:

WC = 35.74 + 0.6215·T − 35.75·V^0.16 + 0.4275·T·V^0.16

Heat index (NWS). A simple formula is tried first: HI = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094). If the average of that result and T is 80 °F or more, the full Rothfusz regression is used instead:

HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²

The NWS adjustments are applied too: a small subtraction when humidity is below 13% (80–112 °F), and an addition when humidity is above 85% (80–87 °F). Celsius inputs are converted to °F and km/h to mph, and the result is converted back.

Worked example

Cold: 0 °F with a 15 mph wind. V^0.16 = 15^0.16 ≈ 1.5423, so WC = 35.74 + 0 − 35.75 × 1.5423 + 0 ≈ −19 °F, which matches the NWS wind chill chart. At 30 °F with a 10 mph wind, the wind chill is about 21 °F.

Hot: 90 °F at 60% relative humidity gives a heat index of about 100 °F (“extreme caution” on the NWS scale), which matches the NWS heat index chart.

FAQ

What is wind chill?

Wind chill estimates how cold exposed skin feels when wind strips heat away from the body. It only applies to people and animals; it doesn't make objects or car engines colder than the air temperature.

When does the heat index apply?

The NWS heat index is designed for temperatures of 80 °F (about 27 °C) and above. It assumes shade and a light breeze; full sun can make it feel several degrees hotter still.

At what wind chill is frostbite a risk?

The NWS wind chill chart shows frostbite possible on exposed skin in about 30 minutes at wind chills around −18 °F (about −28 °C), and faster at lower values.

Why does the tool show no adjustment for some inputs?

Wind chill is only defined at or below 50 °F with wind of at least 3 mph, and heat index at 80 °F and above. Between those ranges the air temperature is the best guide.

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